Geopolymers
Geopolymers are a class of inorganic polymers formed by a polycondensation reaction achieved by alkali activation of an aluminosilicate source or feedstock in a process also known as geosynthesis. After the reaction, the hardened material produced has an amorphous three-dimensional structure that consists of AlO4 and SiO4 tetrahedra linked alternatively by sharing oxygens. These materials, known as poly(sialates), are formed at low temperature and as a result, can incorporate an aggregate skeleton and a reinforcing system if required, during the forming process.
Geopolymers and geopolymerization have been described by Professor Joseph Davidovits in his book entitled Geopolymer Chemistry and Applications, published by Institut Géopolymère in 2008 and in his numerous papers and patents on the subject (Ref: www.geopolymer.org). Geopolymers provide a major shift in perspective, away from the classical crystalline hydration chemistry of conventional Portland cement towards that of organic chemistry and polycondensation reactions.
Geopolymers result from a mineral polycondensation reaction achieved by alkali activation—a process also known as geosynthesis. The term poly(sialate) designates a particular type of geopolymer based on aluminosilicates. The sialate network is an amorphous three-dimensional structure that consists of AlO4 and SiO4 tetrahedra linked alternatively by sharing oxygens. Cations (Na+, K+, Li+, Ca2+ . . . ) must be present in the cavities of the poly(sialate) to balance out the negative charge of the tetravalent aluminium.
The empirical formula of a poly(sialate) is:Mn[(—SiO2)z—AlO2]n·wH2Owhere M is a metal cation (Na, K, Li); n is the degree of polycondensation; and z is the atomic ratio of Si:Al which may be equal to 1 to 32.
The atomic ratios of Si:Al in the poly(sialate) products play an important role in determining the physical properties and applications of the final product. It has been found that materials with a low Si:Al ratio form strong three-dimensional structures suitable for use in construction applications, while at higher ratios the product develops a more linear, two dimensional character and is suitable for use in fire resistant composites. A low Si:Al atomic ratio (1:1, 2:1 or 3:1) results in a rigid three-dimensional network. A ratio above 15:1 results in a more linear, two dimensional material. A ratio of 1:1 can be used for bricks, ceramics and fire protection applications. A 2:1 ratio is suitable for cements and concretes while a 3:1 ratio would be suited for fibreglass composites and tooling for titanium processing.
The three dimensional network poly(sialates) can be summarised as follows:                Si:Al=1:1 (i.e. 1); poly(sialate) or (M)-PS; Mn(—Si—O—Al—O—)n         Si:Al=2:1 (i.e. 2); poly(silalate-siloxo) or (M)-PSS; Mn(—Si—O—Al—O—Si—O—)n         Si:Al=3:1 (i.e. 3); poly(sialate-disiloxo) or (M)-PSDS; Mn(—Si—O—Si—O—Al—O—Si—O—)n         
Depending on the reaction conditions, the poly(sialates) will form an amorphous or semi-crystalline matrix—the later are usually formed in high water, elevated temperature and low Si:Al ratio conditions and the former at ambient temperatures and lower water contents.
The properties and process of manufacture of geopolymer types poly(sialate), poly(sialate-siloxo) and poly(sialate-disiloxo) were the subject of various patents for example French patents FR 2.489.290, 2.489.291, 2.528.818, 2.621.260, 2.659.319, 2.669.918, 2.758.323 or U.S. Pat. Nos. 4,349,386, 4,472,199 and 5,342,595.
A computer molecular graphics of polymeric Mn (—Si—O—Al—O—)n poly(sialate) and Mn(—Si—O—Al—O—Si—O—)n, poly(sialate-siloxo), and related frameworks (Ref 7: Comrie) is shown in FIG. 1(d).
For the description of this invention, the term “geopolymer binder” or “geopolymer cement” relates to a mixture that sets and hardens due to polycondensation. The overall hardening process is known as the “geopolymerization” process. These reactions occur at low temperature and as a result, can incorporate an aggregate skeleton and a reinforcing system if required, during the forming process.
The Geopolymerization Process
Reactants
The reactants consist of the following parts:                1. An aluminosilicate source in the form of a fine powder (typically with a median particle in the range 1 micron to 100 microns). The precursor or feedstock needs to have a significant proportion of its silicon and aluminium held in the correct molecular orientation so as to be rendered reactive when dehydroxylated.        2. An alkali metal hydroxide/silicate solution (often referred to as the alkali activator). The most common activator is an aqueous solution of sodium hydroxide and sodium silicate but other alkali metal systems or mixtures of alkalis can be used. The purpose of the alkali silicate is three-fold: the alkali portion of the solution causes the cleavage of the aluminosilicate precursor; the silicate molecules are involved in the formation of the poly(sialate) and the solution is also the source of the metal cations for charge balancing.        3. A source of calcium such as calcium mellilite present in ground granulated blastfurnace slag to accelerate the polycondensation at room temperature.        
Commonly used precursors include class F Fly Ash, weathered rock where kaolinization is far advanced, calcined clays, aluminium containing silica fume, ground granulated slags or partially calcined clays such as metakaolin, but any fine amorphous aluminosilicate material with Aluminium in IV-V fold coordination with oxygen (as determined by the MAS-NMR spectrum for 27Al) may be used.
Process
As in the case of organic polymerization, the process involves forming monomers in solution and then thermally triggering them, allowing condensation to occur between the reactive groups to form a solid polymer.
The geopolymerization process involves three separate but inter-related stages, namely dissolution, condensation and polycondensation.
1. Dissolution
During initial mixing, the alkaline solution causes the cleavage of siloxo (Si—O—Si—O) chains in the dehydroxylated aluminosilicates present in the feedstock resulting in the formation of the initial monomer necessary for forming the preliminary unit. The precursor powder is the primary feedstock but any amorphous phases present on the surface of the particles of the aggregate skeleton (stone or sand particles) will also react during this stage.
The alkali metal cations from the activating solution are necessary here in order to balance out the charge of the Aluminium in four-fold coordination with oxygen.
2. Condensation
In the solution so formed, neighbouring reactive groups such as Si—ONa and OH—Al along with small silicate molecules from the alkali silicate then undergo a condensation reaction with the liberation of alkali hydroxide to form the preliminary unit for the geopolymerization.
3. Polycondensation
The application of mild heat (typically ambient or up to 90 degrees C.) causes the preliminary units formed in step 2 to polycondense or polymerize, to form rigid chains or nets of oxygen bonded aluminate and silicate tetrahedra.
Higher “curing” temperatures produce stronger geopolymer cements. As each of the hydroxyl groups in these macromolecules are capable of condensing with one from a neighbouring molecule, it is theoretically possible for any one silicon to be bonded, via an oxygen bond, to four neighbouring silicon or aluminium sites, so forming a very rigid polymer network.
Hardened Material Produced
The resultant products are:                a rigid chain or net of geopolymer molecules composed of at least two poly(sialate) types with different Si:Al ratios as discussed previously and        a pore solution composed of water (from the catalytic water initially incorporated in the mix recipe plus water generated as a result of the condensation reactions), excess alkali metal cations and unreacted silicate molecules. In the case of sodium based activators this pore solution can be considered as a weak solution of sodium metasilicate, with a pH of about 12. It forms a continuous nano or meso porosity throughout the geopolymer unless removed during polycondensation.        
The physical properties of the hardened geopolymer are influenced by the Si:Al ratio of the geopolymer network. Below a Si:Al ratio of 3:1, the resultant 3D nets are rigid, suitable as a concrete, cement or waste encapsulating medium. As the Si:Al ratio increases above 3, the resultant geopolymer develops a more linear, two dimensional character and becomes less rigid and more flexible. With higher Si:Al ratios, up to 35:1, the resultant cross linked 2D chains are more suited as an adhesive or sealant, or as an impregnating resin for forming fibre mat composites.
Greenhouse Gases
Wide-scale acceptance of Geopolymer Cements and the concretes they form could reduce the requirement for Ordinary Portland Cement (OPC). This represents a significant opportunity to reduce global carbon dioxide emissions a—                given that the production of OPC requires the calcining of limestone to form the calcium components of OPC, the production of 1 tonne of OPC (by milling OPC clinker) liberates approximately 1 tonne of carbon dioxide to the atmosphere (ref: 1).        global OPC production accounts for about 5 to 10% of worldwide CO2 emissions (ref: 1).        assuming the use of a waste binder such as fly ash and standard chemical activators, the production of 1 tonne of geopolymer cement liberates just 0.16 tonnes of CO2 (ref: 1). The use of waste alkalis would clearly reduce this further.        
The conclusion is that substituting geopolymer cement for OPC would reduce cement generated CO2 emissions by some 80% or more. For total replacement of OPC by geopolymer cement, this potential saving represents some 4 to 8% of current world CO2 emissions.
Geopolymer Composites:
The geopolymer resin for composites is also based on a poly(sialate) Mn(Si—O—Al—O)n structure whose atomic ratio of Si:Al is 3:1 or greater. These composites can be used between 200° and 1000° C. (Ref. 3).
Geopolymer Composites:
High performance fibre composites are based on a two-dimensional crosslinking network with a ratio between 20:1 and 35:1. The working temperature and curing process is dependent on the type of fibre: for E glass it is room temperature for both; for carbon it is <400° C. and room temperature up to 180° C., respectively, for steel it is <750° C. and 80 to 180° C., respectively; and for SiC it is 1000° C. and 80-1800° C., respectively. (Ref. 3).
Composites are made at room temperature or thermoset in a simple autoclave. The advantages of geopolymer composites over organic composites and other materials are: they are easy to make, as they handle easily and do not require high heat; they have a higher heat tolerance than organic composites (carbon reinforced geopolymer composites showed that they will not burn at all, no matter how many times ignition might be attempted); and mechanical properties are similar to those of organic composites. In addition, geopolymers resist all organic solvents (and are only affected by strong hydrochloric acid). (Ref. 3)
It is to be understood that geopolymeric cement is to be distinguished from hydraulic cements also known as Portland cement or Ordinary Portland Cement (OPC). Geopolymeric cements result from a mineral polycondensation reaction by alkaline activation, known as geosynthesis, as distinct from using traditional hydraulic binders in which hardening results from a hydration reaction of calcium aluminates and calcium silicates.
The atomic ratios of Si:Al in geopolymer products play an important role in determining the physical properties and applications of the final product. It is the established view in the art that geopolymer materials with a low Si:Al ratio form strong three-dimensional structures suitable for use in construction applications, while at ratios higher than 15:1, the product develops a linear, two dimensional polymeric character suitable for use in fire resistant composites.
Prior Art Publications
    1. U.S. Pat. No. 7,229,491 granted to Prof. Joseph Davidovits et al, (the disclosure of which is incorporated herein by reference, discloses a geopolymeric cement or binder comprising an amorphous vitreous matrix consisting of a poly (sialate-disiloxo)-type geopolymeric compound, having approximation formula (Na, K, Ca)(—Si—O—Al—O—Si—O—Si—O) or (Na, K, Ca)-PSDS. The inventive cement consists of a mixture of different varieties of polysialates in which the atomic ratio Si:Al varies between 2 and 5.5, the average of the Si:Al atomic ratio values as measured with the electronic microprobe being close to between 2.8 and 3. The remaining components of the geopolymeric cement or binder, such as mellilite particles, aluminosilicate particles and quartz particles, are not used in said Si:Al atomic ratio calculation. The geopolymeric structure of type (K, Ca)-Poly(sialate-disiloxo) (K, Ca)-PSDS is between 50% and 60% more mechanically resistant that that of type (K, Ca)-Poly(sialate-siloxo) (K, Ca)-PSS of the prior art.    2. International Patent Publication No. WO 2008/012438 to Prof. Joseph Davidovits discloses geopolymeric cements based on aluminosilicate fly ash of class F, which, contrary to the prior art, are harmless to use and harden at ambient temperature, favouring their use in common applications in the construction and civil engineering fields. This harmlessnss is achieved thanks to a mixture containing: 10 to 15 parts by weight of a non-corrosive alkali metal silicate solution in which the M2O:SiO2 molar ratio is less than 0.78, preferably less than 0.69, and the SiO2:M2O ratio is greater than 1.28, preferably greater than 1.45, M denoting Na or K; added to this are 10 to 20 parts by weight of water and 5 to 15 parts by weight of blast-furnace slag having specific surface area of less than 400 m2/kg, preferably less than 380 m2/kg and also 50 to 100 parts by weight of class F aluminosilicate fly ash.    3. U.S. Patent Publication No. US 2008/0028994 (Barlet-Gouedard et al) discloses geopolymeric compositions, which have controllable thickening and setting times for a wide range of temperatures and a large range of geopolymer slurry densities. The geopolymer slurry compositions have good mixability and pumpability, whilst the set materials develop good compressive strength and permeability. The invention discloses a method for preparing geopolymer for oilfield cementing applications. The geopolymeric compositions according to the invention comprises a suspension comprising an aluminosilicate source, a metal silicate, an alkali activator, lightweight or heavyweight fillers and a carrier fluid wherein the suspension of said geopolymeric composition is pumped in a well and allowed to set.
The geopolymeric compositions disclosed in US 2008/0028994 are preferably poly(silate), poly(sialate-siloxo) or poly(sialate-disiloxo). More preferably, the geopolymeric composition are poly(sialate-siloxo) components and therefore the silicon to aluminium atomic ratio is substantially equal to two, between 1.8 and 2.2.
This is a pumpable composition for use in the oilfield industry having a particular rheology [see column 2, paragraph [009] of US 2008/0028994].    4. U.S. Pat. No. 6,869,473 discloses cementicious materials including stainless slag and geopolymer can be added to conventional cement compositions, such as Portland cement, as a partial or total replacement for conventional cement materials. The stainless steel slag may comprise silicates and/or oxides of calcium, silicon, magnesium, iron, aluminium, manganese, titanium, sulphur, chromium and/or nickel. The geopolymer may comprise aluminium silicate and/or magnesium silicate.
The present invention does not involve inclusion of any conventional cement compositions, such as Portland cement in the geopolymer composition.    5. German Patent Publication No. DE 19535390 discloses that outer walls, inner walls, floor ceiling or roofs are constructed from bar-shaped building members of concrete or a geopolymer foam material. In cross-section, the members have the shape of an equilateral triangles and they are joined together with U-shaped recesses in the basal surfaces of the concrete members. The recesses can be filled with an appropriate material, such as thermal insulation. Members are joined with an adhesive, which is one of the materials of at least one building member.Geopolymer CementPrior Art
There are a number of geopolymeric cements of the prior art (WO 92/04298, WO 92/04299, WO 95/13995, WO 98/31644, U.S. Pat. No. 4,509,985) which are the result of a polycondensation between an aluminosilicate, potassium or sodium disilicate and calcium disilicate. When potassium disilicate is used the obtained geopolymer is of the type (K, Ca)-poly(sialate-siloxo).
There are also geopolymeric cements of the prior art (WO 03/099738) which are the result of hardening of a mixture of a calcined strongly weathered granitic type rock in which kaolinisation is far advanced, calcium mellilite glass and a soluble alkaline silicate.
The prior art relating to the manufacture of a geopolymer cement has focused on the use of pure metakaolin or calcined weathered granitic type rock with an Si:Al atomic ratio of no less than 1:1. Further, there has been research carried out which found that precursors with a high alumina content would result in a low compressive strength material with little geopolymeric properties. (Ref: The composition range of aluminosilicate geopolymers. Ross A. Fletcher, Kenneth J. D. MacKenzie, Catherine L. Nicholson and Shiro Shimada).
The present invention seeks to alleviate the disadvantages associated with the prior art. In particular, the present invention provides a geopolymer cement produced from a precursor material having a relatively high alumina content which surprisingly, in accordance with the present invention, produces structural building units having relatively high compressive strength.
The present invention uses aluminosilicate geopolymer technologies in novel ways. These aluminosilicate geopolymers are synthesised at low temperatures from a variety of mineral and or organic precursors and an alkali reagent.